1. Field of the invention
This invention relates to optical disk drive mirror signal arithmetic circuits, and more particularly to an optical disk drive mirror signal arithmetic circuit in which a mirror signal is eliminated which is due to a main beam leakage signal to a sub photo diode.
2. Related art
As shown in FIG. 2, a Light receiving section of a three-spot beam pickup employed for an optical disk drive comprises: a main photo diode 1 at the center; and right and left photo diodes provided on both sides of the main photo diode 1. The main photo diode receives a main beam though a cylindrical lens, and has a light receiving surface which is radially divided into four parts (A, B, C and D). A focus servo mechanism utilizes the difference between the outputs of two pairs (A and C, and B and D) in each of which two light receiving surfaces are confronted with each other through the optical axis), controls the focus of the objective lens, and uses a decoder to decode the sum signal of main beams received, and a control section reads a digital signal.
A tracking error signal is produced owing to the difference between the quantities of received light of sub beams SB of the right and left sub photo diodes 2 and 3, and a tracking servo mechanism controls a tracking actuator to position the main beam on the data track of the optical disk.
In order to count the number of tracks which have passed at the time of random access, a mirror signal V.sub.MIRR adapted to detect a mirror surface section of the optical disk is calculated from a current-voltage-converted main photo diode sum signal V.sub.MAIN and a sub photo diode sum signal V.sub.SUB, and it is determined from the voltage level of the mirror signal V.sub.MIRR whether the main beam is on the data track groove of the optical disk, or whether it is on the mirror surface section between tracks. EQU V.sub.MIRR =V.sub.MAIN -k*V.sub.SUB Equation (1)
where
V.sub.MAIN =V.sub.A +V.sub.B +V.sub.C +V.sub.D PA1 V.sub.SUB +V.sub.E +V.sub.F +V.sub.G +V.sub.H PA1 k=ratio of the quantity of light of the main beam to that of the sub beam
However, as indicated in a main beam quantity-of-light distribution graph of FIG. 2, at the time of random access the periphery of the main beam MB reflected from the data track groove of the optical disk is applied to the sub photo diodes 2 and 3, as a result of which sometimes the sum signal of the sub photo diodes may be V.sub.SUB E which includes the incidence light V.sub.SUB of the sub beams SB, and the leakage signal V.sub.err of the main beam. EQU V.sub.SUB E =(V.sub.E +V.sub.F V.sub.G +V.sub.H)+(V.sub.E err +V.sub.F err +V.sub.G err +V.sub.H err)
The sub photo diode main beam leakage signal V.sub.err causes no trouble in the tracking servo that a tracking control signal is formed according to the difference between the outputs of the right and left sub photo diodes, because it is canceled out by subtraction; however, in the case of the mirror signal V.sub.MIRR adapted to obtain the sum of the outputs of the right and left sub photo diodes, it becomes an error, as a result of which a track count error occurs; that is, it becomes impossible to achieve a seek operation.
This gives rises to a problem that, in the optical disk device, the track count error is technically eliminated, whereby the seek operation is improved in stability. Accordingly, an object of the invention is to solve the problem.